Remove -Z when GOLFVIM=nvim

[AlexApps99]

See #304

This PR conditionally removes the -Z flag when $GOLFVIM is set to nvim.

This may make it slightly easier to cheat, but it doesn't really change that much considering that you can make a shellscript for $GOLFVIM that cuts out the first arg pretty easily. (exec nvim "${@:2}")

I also changed the diff tool to use the provided vim by default, since most people that change GOLFVIM would expect vimgolf to use it for diff too.

Making the experience for nvim slightly better.

[AlexApps99]

For good anti-cheat/validation, it might make sense to validate the key presses on the server (no idea how that'd work, unlikely that would ever be implemented, just putting it out there)

[jenvigo]

I would love to see this PR merged.

It worked perfectly for me.